Geology of Fuji Volcano

Eruptive history of Fuji Volcano

Fuji Volcano is a large composite stratocone consisting of alternating
lava flows and pyroclastics. It is composed of three cones, Komitake, Older
Fuji and Younger Fuji volcanoes, in order of decreasing age. Borehole data
revealed that the basement rocks beneath the volcano belongs to the Misaka
Group, which is composed mainly of Tertiary marine sediments. Moreover,
accidental fragments from Tertiary volcanic rocks were also found in Fuji
Volcano's ejecta.

The activity of Komitake Volcano started in the middle Pleistocene with
the eruption of andesite lava flows. This activity is coeval with Ashitaka
Volcano located on the southeastern foot of Mt. Fuji. The volcanic center
of Komitake Volcano is interpreted to be at the northern flank of Mt. Fuji
as indicated by the slopes of lava flows comprising Komitake Volcano.

Older Fuji Volcano started its activity 80 ka and ended 11 ka. Its activity
was characterized by the ejection of voluminous pyroclastic falls. Total
volume of the ejecta is estimated as 250 km^3. Large-scale mudflows (Older
Fuji mudflows), generated from pyroclastic-flows and debris-avalanche deposits,
also characterizes the older volcano.

Eruptive history of Younger Fuji Volcano

The Younger Fuji Volcano exhibited various types of eruptions during 11,000
years. The eruptive history of the Younger Fuji was studied by Tsuya (1968),
Machida (1964), Uesugi et al., (1980), and Uesugi (1990). Tsuya (1968)
divided the Younger Fuji into three stages, older, middle, and younger,
and Miyaji (1988) furthermore gave the detailed eruptive history tephrochronologically,
as follows.

Distribution of flank (parasitic) cones and craters of Fuji Volcano. The
parasitic cones and craters are lining up in the direction of NW-SE.

(1) 11 - 8 ka
Summit and flank fissure eruptions produced large volumes of lavas,
mostly of olivine basalt. They are distributed mainly on the northern,
western and southwestern foot. Small-scale tephra eruptions were also associated.(2) 8 - 4.5 ka
Voluminous lava eruption was followed by an intermittent small-scale
ejection of tephra. These tephras deposited thickly on the flank and eastern
foot of volcano. The felsic ash layer that erupted 6.4 ka at Kikai Caldera
(southern Kyushu) is found in them. (3) 4.5 - 3 ka
Major lava flows and minor pyroclastic falls erupted from the summit and
flank craters. Most of lava flows are blocky, and pyroxene olivine basalt.(4) 2 - 3 ka
The activity became explosive and major pyroclastic falls were erupted
intermittently, mostly from the summit crater. Pyroclastic flows also occurred
in this stage; the deposits were found on the southern and western slopes
of the volcano. AT 2.5 ka, a large-scale debris avalanche, the Gotemba
Debris Avalanche, generated and attacked on the eastern foot of volcano.(5) 2 ka to 1707 AD
Strombolian activity occurred at the NW-SE trending flank craters, formed
many scoria cones. Most of the lava flows are "aa" lava of pyroxene
olivine basalt. In the 800-864 eruption, major lava flows effused from
a parasitic cone at the NW flank of volcano. A large lake was partially
buried by lava flows and separated into two lakes, the present lakes of
Sai and Shoji. (6) 1707 AD
On December 16, 1707 (10h00m), a large plinian eruption occurred at the
SE flank of volcano. Ashes dispersed eastward (toward the Edo, presently
Tokyo), causing darkness even around 13h00m. Even in the night heavy ash
fall continued in the area east of the volcano. Tephras changed the chemistry
with time from dacitic to basaltic missing andesitic composition. Total
volume of tephra reached about 1.7 km^3. After this eruption, no signs
of renewed activity were recorded, except for some steaming at the summit
vent during 1780-1820.
(originally written by N. Miyaji)